1. Field of the Invention
The present invention relates generally to apparatus and methods for reducing the magnitude of electric field emissions, and more specifically to an AC electric field cancellation system and method for reducing the magnitude of time varying electric field emissions from a CRT display.
2. Brief History of the Art
Cathode Ray Tube (CRT) displays are used in conjunction with a wide variety of information and entertainment systems, and are most commonly associated with ordinary television sets and computer monitors. These displays normally include complex electronic circuitry which generates a substantial number and variety of complex alternating electromagnetic fields. A significant portion of these electric fields emanate towards and through the CRT screen, rear and sides of the display, eventually reaching the display operator and those in the proximity of the display.
In conventional color displays, the shadow mask, aluminum coating on the back of the phosphor, and the CRT's internal magnetic shield all play an important role in the electric field profile of the monitor, often serving as radiators of the internal electric fields. For example, in displays with an integrated high voltage horizontal deflection circuit, the anode voltage may be modulated by side pin cushion correction. The resulting parabolic voltage wave form is integrated by the flyback transformer driving impedance and the anode capacitance, typically resulting in an approximately tens of volts, peak to peak, of S shaped or sometimes parabolic wave form. Peak to peak video current from the cathode generates a voltage drop across the anode impedance, which is determined by the aquadag capacitance and whatever bleeder resister may be present. The result is low passed video on the anode which may be super-imposed on the S or parabolic wave form, if present, as described above. When the worst case image is presented (one half screen white and one half screen black), the anode voltage can fluctuate up to several hundred volts, peak to peak. Thus stimulated, the shadow mask, the aluminum coating on the back of the phosphor, and the CRT's internal magnetic shields radiate AC electric fields.
Another source of electric fields, particularly in the very low frequency range, is the flyback pulse from the horizontal deflection circuit. This pulse ranges from several hundred volts for some monochrome displays to over a kilovolt for color displays. Additionally, at extremely low frequencies, the vertical deflection circuit, power mains wiring, and if present, the degaussing coil often contribute to the voltage fields.
During the past several years major concerns have been raised by various groups regarding potential health hazards inherent in devices which generate electric fields. Although there are presently no U.S. government regulatory standards defining harmful vs. non-harmful levels of electric field exposure, several international communities have discussed potential guidelines. Increased public awareness of these potential hazards has surfaced, and has led to an increasing number of products designed to limit the intensity of electric field emissions.
To address these concerns, display manufacturers have integrated a variety of countermeasures into the display design. Conventional methods of reducing the magnitude of the electric field from a CRT display mainly consist of various types of shielding. For example, an existing counter measure for reducing the frontally directed AC electric field includes placing a conductive screen or thin metal film over the CRT face. The screen or film must then be electrically connected to chassis ground. Although this and other counter measures help in reducing overall field emissions, there are several serious inherent problems which limit their effectiveness. These problems include degradation of front of screen CRT performance, mechanical form and fit problems, increased internal temperature due to restricted air flow, increased unit weight, increased power dissipation and geometric distortion of the display image, and high associated design and production costs. Thus, the existing solutions for reducing the AC electric field emissions from a CRT display are inadequate.